This invention relates to a method of forming a thermoplastic layer on a layer of adhesive.
Image graphics are omnipresent in modem life. Images and data that warn, educate, entertain, advertise, etc. are applied on a variety of interior and exterior, vertical and horizontal surfaces. Nonlimiting examples of image graphics range from posters that advertise the arrival of a new movie to warning signs near the edges of stairways.
A surface of an image graphic film requires characteristics that permit imaging using at least one of the known imaging techniques. Nonlimiting examples of imaging techniques include solvent based inks, 100% solids ultraviolet curable inks, water based inkjet printing, thermal transfer, screen printing, offset printing, flexographic printing, and electrostatic transfer imaging.
Electrostatic transfer for digital imaging employs a computer to generate an electronic digital image, an electrostatic printer to convert the electronic digital image to a multicolor toned image on a transfer medium, and a laminator to transfer the toned image to a durable substrate. Electrostatic transfer processes are disclosed in U.S. Pat. No. 5,045,391 (Brandt et al.): U.S. Pat. No. 5,262,259 (Chou et al.); U.S. Pat. No. 5,106,710 (Wang et al.); U.S. Pat. No. 5,114,520 (Wang et al.); and U.S. Pat. No. 5,071,728 (Watts et al.), the disclosures of which are incorporated by reference herein, and are used in the SCOTCHPRINT(trademark) electronic imaging process commercially available from 3M.
Nonlimiting examples of electrostatic printing systems include the SCOTCHPRINT(trademark) Electronic Graphics System from 3M. This system employs the use of personal computers and electronically stored and manipulated images. Nonlimiting examples of electrostatic printers are single-pass printers (Models 9510 and 9512 from Nippon Steel Corporation of Tokyo, Japan and the SCOTCHPRINT(trademark) 2000 Electrostatic Printer from 3M) and multiple-pass printers (Model 8900 Series printers from Xerox Corporation of Rochester N.Y., USA and Model 5400 Series from Raster Graphics of San Jose, Calif., USA).
Nonlimiting examples of electrostatic toners include Model 8700 Series toners from 3M. Nonlimiting examples of transfer media include Model 8600 media (e.g., 8601, 8603, and 8605) from 3M.
Nonlimiting examples of laminators for transfer of the digital electrostatic image include Orca III laminator from GBC Protec, DeForest, Wis.
After transfer of the digital electrostatic image from the transfer medium to a film or tape, optionally but preferably, a protective layer is applied to the resulting imaged film or tape. Nonlimiting examples of protective layers include liquid-applied xe2x80x9cclearsxe2x80x9d or overlaminate films. Nonlimiting examples of protective clears include the Model 8900 Series Scotchcal(trademark) Protective Overlaminate materials from 3M. Nonlimiting examples of protective overlaminates include those materials disclosed in U.S. Pat. No. 5,681,660 (Bull et al.) and copending, coassigned, PCT Pat. Appln. Ser. No. US96/07079 (Bull et al.) designating the USA and those materials marketed by 3M as SCOTCHPRINT(trademark) 8626 and 3645 Overlaminate Films.
Thermal ink jet hardware is commercially available from a number of multinational companies, including without limitation, Hewlett-Packard Corporation of Palo Alto, Calif., USA; Encad Corporation of San Diego, Calif., USA; Xerox Corporation of Rochester, N.Y., USA; LaserMaster Corporation of Eden Prairie, Minn., USA; and Mimaki Engineering Co., Ltd. of Tokyo, Japan. The number and variety of printers changes rapidly as printer makers are constantly improving their products for consumers. Printers are made both in desk-top size and wide format size depending on the size of the finished graphic desired. Nonlimiting examples of popular commercial scale thermal ink jet printers are Encad""s NovaJet Pro printers and H-P""s 650C and 750C printers. Nonlimiting examples of popular desk-top thermal ink jet printers include H-P""s DeskJet printers.
3M markets Graphic Maker Ink Jet software useful in converting digital images from the Internet, ClipArt, or Digital Camera sources into signals to thermal ink jet printers to print such images.
Ink jet inks are also commercially available from a number of multinational companies, particularly 3M which markets its Series 8551; 8552; 8553; and 8554 pigmented ink jet inks. The use of four principal colors: cyan, magenta, yellow, and black permit the formation of as many as 256 colors or more in the digital image.
Current image graphic films contain vinyl chloride polymers, such as marketed by 3M under the SCOTCHCAL(trademark) brand. Alternatively, multilayer films such as disclosed in U.S. Pat. No. 5,721,086 (Emslander et al.) can be used for reception of image graphics. In both instances, specialized coatings are used as the receptor surface on an underlying substrate to improve image graphics transfer and image quality. Regardless, both types of image graphic films have an adhesive layer (and protective release liner until use) on the opposing surface of the film substrate. Thus, image graphic films currently are laminates of some specialized coating, a substrate, an adhesive, and a release liner until use.
In another art, powder coating typically involves applying a specially formulated powder to a substrate by one of several known techniques and then heating the powder in an oven in order to cause the powder to melt and flow to form the coating. The process may also include a curing step to allow a chemical reaction to occur in the coating. The result is a coating with desirable visual and functional properties. A primer may be required to achieve adequate adhesion to the substrate. This method is generally used with metal or heat resistant plastic parts because of the high temperatures that are necessary to achieve complete melting and flowing of the powder. Polymers used in powder coatings typically have a relatively low viscosity when melted so that the powder will be able to form a continuous film under the applied heat. While powder coating is a solvent-free process, it generally requires significant oven cycle times and large, energy-intensive ovens.
A common method of producing polymeric powders for powder coating is to melt and mix the desired resins in a twin screw extruder, extrude and cool the polymer mass and grind the mass to a desired size. The resulting powder, when viewed microscopically, has irregularly-shaped particles with sharp, pointed edges. These particles may exhibit low packing density when deposited on a substrate, resulting in a coating that is susceptible to voids. The irregular shapes also do not achieve the maximum charge to mass ratio as noted in U.S. Pat. No. 5,399,597 that is desirable for certain types of powder coating.
The present invention has addressed a problem not recognized by the prior art, namely: that image graphic films need not have a film substrate to provide structural integrity between the thermoplastic film and the adhesive, if the thermoplastic film can be formed directly on the adhesive.
The present invention has solved the problems in the art by developing a method of forming a thermoplastic layer on an adhesive layer by powder coating without the use of solvents. The method can be successfully practiced with combinations of polymers that may be chemically incompatible or unstable in processing systems such as emulsions or latices. The method provides a shortened and simplified manufacturing process by avoiding long curing ovens and convoluted web lines, instead relying on the combined application of heat and pressure to the coated substrate. The absence of solvents in the process means that capital costs for scrubbing equipment and special ventilation systems are eliminated, along with the environmental effects associated with solvent coating.
In one aspect, the present invention provides a method of forming a thermoplastic layer on an adhesive layer having two major opposing surfaces. The method comprises the following steps: a) providing a thermoplastic powder having a melt flow index of at least about 0.008 grams/10 minutes; b) applying the powder to at least one major surface of the adhesive layer to form a particle layer; and c) subjecting the particle layer of step b) to elevated heat an pressure until the powder in the particle layer is fused into a continuous layer and the continuous layer is bonded to the adhesive layer. The melt flow index of the powder is preferably in the range from about 0.008 grams/10 minutes to about 50 grams/10 minutes.
As used herein, xe2x80x9cmelt flow indexxe2x80x9d refers to a measure of the rate of polymer melt flow through a capillary and is measured at 190xc2x0 C. according to ASTM Method D-1238 for polypropylene. The reported index is the average of three measurements. A lower melt flow index indicates a slower-flowing, more viscous polymer that is likely to be relatively high in molecular weight.
xe2x80x9cFusedxe2x80x9d means that the powder particles have melted at least partially and have joined with adjacent powder particles sufficiently to form a continuous layer.
xe2x80x9cJoinedxe2x80x9d means that adjacent powder particles no longer have a distinct boundary layer when viewed under magnification.
xe2x80x9cContinuousxe2x80x9d means that the layer covers or surrounds the entire substrate with substantially no gaps or pin holes having a size greater than is considered acceptable for a particular application. It is not required that the continuous layer be a completely homogeneous film. The continuous layer may be formed from a monolayer of particles, or from more than one layer of stacked particles.
xe2x80x9cBondedxe2x80x9d means that the bond strength between the continuous layer and the substrate is greater than the internal tensile strength of the weaker layer.
The term xe2x80x9cthermoplasticxe2x80x9d refers to materials that soften and flow upon exposure to heat and pressure. Thermoplastic is contrasted with xe2x80x9cthermosetxe2x80x9d, which describes materials that react irreversibly upon heating so that subsequent applications of heat and pressure do not cause them to soften and flow.
xe2x80x9cTwo-dimensionalxe2x80x9d with reference to the substrate means that the substrate is a sheet having two major opposing surfaces that is capable of passing through a nip roll configuration.
For this invention, the application of heat and pressure is preferably accomplished by passing the coated substrate through a heated nip roll configuration using readily available equipment. One skilled in the art can choose thermoplastic powder compositions that will yield useful thermoplastic layers having a variety of properties, such as dirt and stain resistance, ink and graphics receptivity, and porosity.
In another aspect, the present invention provides a composite sheet material comprising an adhesive layer having two major opposing surfaces and a thermoplastic layer overlying and bonded to at least one major surface of the adhesive. The thermoplastic layer is continuous and comprises a fused thermoplastic powder. The powder has a melt flow index ranging from about 0.008 grams/10 minutes to about 50 grams/10 minutes, and preferably about 1 grams/10 minutes to about 35 grams/10 minutes. Preferably, the composite sheet material is useful as an outdoor sign and the powder comprises a ionomer or a vinyl chloride polymer.
A feature of the invention is low profile of the composite sheet material because of the elimination of the film substrate that was previously provided for structural integrity rather than for imaging.
An advantage of the invention is the reduction in cost of the composite sheet material because of the elimination of the film substrate and the attendant production steps to make that film substrate.
Another advantage of the invention is the lower profile of the composite sheet material results in a more conformable, more receptive image graphic film due to the absence of the film substrate and the softness of the combination of the thermoplastic layer and the adhesive layer.
Another advantage of the invention is the avoidance of pollution abatement equipment because the method of the invention is a solventless process.
Another advantage of the invention is the method of the present invention avoids the use of extrusion processes where the possibility of the extrusion head contacting the adhesive layer is problematic to error-free processing.
Another advantage of the invention is the use of a powder coating process to prepare a continuous layer of a thermoplastic film on an adhesive layer which provides good dimensional stability in the thermoplastic film, because such film is formed without polymeric orientation inherent in extrusion processes.
Another advantage of the invention is that the method uses no thermal oxidizer, providing lower operating cost to make the thermoplastic film via powder coating processes.
Embodiments of invention are further described with reference to the following description.